Pulse keying system



Sept- 8, 1953 E. M. BUYER 2,651,753

PULSE KEYING SYSTEM Filed Oct. 2, 1952 POSITIVE TIMING KEY/N6 CKT. PULSEI GEN.

SIGNAL A SOURCE CUTOFF ems 8 SIGNAL PULSE TIMING c102 f I f- DELAY "'i 1I i 25 KEY/N6 PULSE VOL TAG CATHODE (/6) i POTENTIAL 25 i I ourrur AT(I8\36 0 POSITIVE (if 5 smc. KEYING cKT. PU-ZE 6 38 5:32 0 a5 is 7 F45 FtP05IIVE 3 0 75 r PULSE 'ZZ TEST DELAY 3/ A KEY/N6 PULSE LuPSYMZCKTIBDDELAY N R Es I TEST P1155 .emh /wguym 5 l RECIRLULATED B cPULSES 7 I DELAYNETKiQ T ATTORNEY Patented Sept. 8, 1953 PULSE KEYINGSYSTEM Edward M. Buyer, Ramsey, N. J., assignor to FederalTelecommunication Laboratories, Inc., Nutley, N. J a corporation ofDelaware Application October 2, 1952, Serial No. 312,696

8 Claims.

This invention relates to pulse keying systems and particularly to apulse keying system which can be used for testing of multihop radio linkequipment.

The main purpose of the invention is to provide a novel system fordistortionless interuption of a closed loop transmission circuit, suchas a television or multichannel telephone transmission system at anarbitrary instant of time.

Since keying usually causes the pulses sent out on the transmissionsystem by the keyed stages to change abruptly, said abrupt changes mayproduce transients and thus disturb the shape and character of thetransmitted pulses, particularly if the keying speed is high.

Prior art pulse keying circuits attempt to balance out said transientbut cannot achieve its perfect cancellation.

Therefore, an object of this invention is a circuit for keying a systemwithout introducing an initial switching transient.

A further object of the invention is to provide a system which may beused for testing of multihop radio link equipment.

A feature of the system is that it permits simulation of a longtransmission circuit involving many repeaters by using only a single ortwo hops of such circuits. This is acomplished by recirculating a signalthrough one or two hops in conjunction with the keying circuit of thisinvention.

The above-mentioned and other features and objects of this inventionwill become more apparent by reference to the following descriptiontaken in conjunction with the accompanying drawings, in which:

Fig. 1 is a schematic circuit diagram of a keying system according tothe invention;

Fig. 2 shows waveforms useful in explaining the operation of the circuitshown in Fig. 1;

Fig. 3 is a circuit diagram of the circulating pulse arrangementaccording to the invention; and

Fig. 4 shows the waveform sequences useful in explaining the circulatingpulse arrangement shown in Fig. 3.

Referring to Fig. l, a signal source I has coupled thereto a, timingcircuit 2, providing appropriate pulse delays, and a synchronizedpositive keying pulse generator circuit 3, all such circuits beingsuitable conventional type. The synchronized positive keying pulsesproduced by generator 3 are impressed on suppressor grid 4 of amulti-electrode tube 5, through capacitor 6. A source of the cut-offbias voltage 8 is connected across a resistor l to the suppressor grid4.

The signal source I is also connected to the control grid 9 of the tube5. The voltage source is applied at the anode ll of the keying tubeacross a resistor [2. A diode vacuum tube H or other directional elementis connected in shunt and another diode or directional element [5 isconnected in series with the output it of tube 5 as follows: The anodell of tube 5 is coupled to the anode l4 of the diode l5 and to thecathode I6 of the diode ll by a capacitor l3. The cathode I9 of the tube5 and the anode 28 of shunt diode H are connected to ground. The cathode2| of diode I5 is connected to the output terminal I8 and also to groundthrough a resistor 22.

The keying circuit operates as follows: First, for proper signaltransmission through the keyer, the positive keying input voltage fromgenerator 3 must have a flat maximum or minimum, as for example, arectangular pulse 23. The keyer is triggered by a negative input signalpulse 24 from source I during this period when the positive input keyingvoltage 23 is not changing. Tube 5 is normally cut-off by the suppressorgrid bias, and the keying pulse 23 causes conduction in the tube 5. Whenthe plate voltage applied at I0 drops due to conduction of tube 5, thecapacitor l3 discharges through the shunt diode l'l until the voltageacross capacitor l3 equals the plate voltage of the tube 5. Thisproduces negative pulse 25, curve C, Fig. 2, at the cathode IS. Thenegative pulse 25 at the cathode 16 of the diode ll, however, is nottransmitted through the diode i5 since the plate i i of the latter isnegative with respect to its cathode 2|. Hence the leading edge of thekeying pulse 23 does not appear at the output 18.

The shunt diode H is used to bring the charge on the capacitor 3 toequilibrium very quickly after the leading edge of the keying pulse 23ap pears without affecting the low-frequency response of the circuitduring the signal period. This is shown by the portion 26 of curve C,Fig. 2. As shown by the waveforms of Fig. 2, the circuit is keyed topass alternate pulses from source I, the curve A being a waveform of thesignal pulses of pulse source I. Curve B is a waveform of keying pulsefrom generator 3 showing the timing delay produced by the timing circuit2. Curve C is a waveform in the cathode it of diode l1, and curve D is awaveform of the output at 18. The negative test pulse 24 is applied atthe control grid of tube 5 while plate current is flowing due to pulse23. The test pulse hence appears as a positive signal at anode ii. DiodeI! does not conduct at this time since its cathode IB is positive, andthe test pulse passes through diode IE to the output 18.

At the termination of pulse 23, the anode H voltage rises and a positiveoutput pulse is passed through diode [5 to the output 58. This outputpulse 28 decays along curve 29 as capacitor i3 becomes charged throughthe path consisting of resistor I2, capacitor l3, diode l5 and resistor22. It can be seen that no leading edge keying transient appears in thewave D. This method is particularly useful where the trailing edgetransient is not important, as is often the case.

An important application of this invention is in a video circulatingpulse system, where pulses are allowed to recirculatc through anamplifier in order to predict the performance of any of such amplifierscascaded in a relay system. A brief description of its principles, withreference to the circuit diagram of the circulating pulse arrangementshown in Fig. 3 wherein an amplifier 30 is under test, will make clearthis application of the invention. The main elements of this applicationcircuit are easily recognized as parts of the original circuit, shown inFig. l, the corresponding elements of the two figures bein given likereference characters. The delay network .3! introduces a delay greaterthan the width of the test pulse 32 of source 33 to prevent the leadingportion of a pulse from overlapping the trailing portion of thepreceding pulse. The attenuator 35 reduces the loop gain to unity sothat each pulse will be of equal amplitude. The positive keying pulse 35of generator 36 is synchronized to the test pulse 32 through delaycircuit '3"! similarly as hereinbefore described. When the test pulsehas circulated the desired number or" times, the system is cut off. Inorder to view the pulse shapes resulting from the test pulse circulated,an oscilloscope 38 is synchronized with the keying pulse 35, therebyproviding a stationary picture of a train of pulses circulating through.the sys tem. Recirculation of the test pulses 32 as output pulses M isobtained by providing an input coupling stage 39 having a vacuum tube 40similar to tube 5. The tube 40 is provided with a control grid ll towhich the test pulse 32 is applied, while the output pulse 44 or" theloop 36, 3 Si is applied to the suppressor grid 42 thereof. The outputpulses 43 of tube 60 are negative and are applied as the input pulse totube 5 together with the keyer pulse 35 the same as described inconnection with the circuit of Fig. 1.

4 shows the waveform sequences in the above-described circulating pulsesystem. The leading edge of the keying pulse is eliminated as in Figs. 1and 2 thus avoiding circulation of a corresponding transient. The seriesdiode l5 accomplishes this. The pulse 32 which first triggers thecircuit in coincidence with keying pulse 35 will first appear on theoscilloscope 38 .as pulse it as shown in curve C, Fig. 4. Since thepulse 5 is also recirculated, it will produce a second pulse d5 on theoscilloscope delayed an amount corresponding to the delaycharacteristics of the network 3| plus the time consumption of theremaining circuit elements or the loop. The retention characteristics ofthe oscilloscope is such that a series of recirculating pulseindications will be shown simultaneously on the screen substantially asindicated in curve C, the number or" such pulse indications 44, 45,-etc., depending upon the recirculation cycles of an input test pulse,or until termination of keyer pulse 35. If the amplifier or othercircuit under test is iunctioning satisfactory, the pulse series it, 45,etc., of curve C will show uniformity, and if it is not functioningproperly, the pulse series will show distortion, the degree ofdistortion increasing for each successive pulse circuluated.

While I have described above the principles of my invention inconnection with specific apparatus, it is to be clearly understood thatthis description is made by way of example only and not as a limitationto the scope of my invention 4 as set forth in the objects thereof andin the accompanying claims.

I claim:

1. In a ulse keying system, an electron discharge device, means tonormally bias said device to cut off, a capacitor coupled to the outputof said device, a pair of unidirectional elements, one coupled in serieswith said output and the other connected in shunt with said output,means to apply a keying voltage having a substantially fiat voltageportion to the input of said device to overcome said out off bias, andmeans to apply a test pulse of short duration to the input of saiddevice in coincidence with the flat voltage portion of said keyingvoltage, the shunt coupled element serving to stabilize the chargedcondition of said capacitor following application of the leading edge ofsaid keying voltage and said series coupled element serving to pass onlythose variations of a given polarity relative to said stable chargedcondition.

2. In a pulse keying system according to claim 1, wherein the means forapplying a keying voltage includes a pulse generator and means forsynchronizing the output of said pulse generator with the occurrence ofsaid test pulses.

3. In a pulse keying system according to claim 2, wherein said deviceincludes two grid electrodes and the means for applying said keyingvoltage is coupled to one of said grid electrodes and the means forapplying the test pulse is coupled to the other of said grid electrodes.

4. In a pulse keying system according to claim 1, wherein said capacitoris coupled between the anode of said electron discharge device and saidunidirectional elements, said elements each having an anode and acathode, one side of said capacitor being coupled to the cathode of saidshunt coupled element and to the anode of said series coupled element.

5. In a pulse keying system according to claim 1, further includingmeans for recirculating the output of said series coupled elementthrough said keying system, including means for applying the output ofsaid series coupled element to said electron discharge device.

6. In a pulse keying system according to claim 5, wherein said means forrecirculating the output of said series coupled element includes meansfor imposing a delay of a given time interval to such output.

7. In a pulse keying system according to claim 5, wherein said means forrecirculating the output of said series coupled element includes acircuit, the operating characteristics of which are to be tested, andmeans for displaying the pulses circulated to indicate the operatingcharacteristics of the circuit under test.

8. In a pulse keying system according to claim 5, wherein the means 'forrecirculating the output of said series coupled element includes acircuit the operation of which is to be tested and an attenuator forevening up the pulses recirculated, and means for delaying the pulsescirculated by a given time interval, an oscilloscope, means tosynchronize said oscilloscope with said keying voltage and means forapplying the recirculating pulses to said oscilloscope after passingthrough the circuit under test.

EDWARD M. BUYER References Cited in the file of this patent Beck,Proceedings of the Institute of Radio Engineers, vol. 35, No. 11,November 1947, pages 1226-1230.

